Jailbreak

Cydia

NOTE: There is an application similar to this for the PC that is in BETA and is already available called iDesigner. More geared for designers than the avid iPhone user though. Read more [ame="http://www.ipodtouchfans.com/forums/showthread.php?t=245104"]here[/ame]
UPDATE: using Mono (Main Page - Mono) you can run iDesigner on linux and even Mac

I caught wind today of an Windows Application that is in development by a the folks over at BeYourIPhone and a user by the name of AleBiT, that is designed to let you preview WinterBoard themes on your PC before installing them to your iPhone.

Don't get your hopes up just yet. Although the application is still in development (it looks like it still needs MAJOR development work, and it's not even in english, let alone its not exactly the most stable over at beyouriphone), this will be a more than useful tool for those of us with Jailbroken iPhones and a large amount of WinterBoard Themes. I can spend an hour just browsing themes and trying them out, so this could save a lot of time from respringing my iPhone a gazillion times.

This is also good news for Theme creators. I've personally been working a theme in development called Elemental, and when I test out a new set of icons I can end up respringing my phone enough times to want to bash my head into the wall.

Keep your eyes peeled for this one and possibly some others, hopefully one for us Mac users. It's too bad the iPhone simulator didn't sync with the iPhone. Would be PERFECT for theme testing.

Not sure how often this will be updated but you can check out the developers blog here: AleBiT Dev Blog